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Wednesday, July 3, 2013

Digestive System, Part 4: B-12 as Temptress

Have you ever lost anything, Homo sapien? Your purse? Your car keys?

Well, it’s rather like that. Now you have it, now you don't.

We lost a vitamin that’s critical to our survival. And good luck trying to find it now. The last time anyone can remember having it was at least 2 billion years ago. Yes, that's "billion" with a ba-ba-billion.

It's vitamin B-12. We need it to live, but we can't make it ourselves.In fact, the only naturally occurring creatures that can make vitamin B-12 are bacteria and archaea, the simplest organisms on Earth.1

So how the hell do animals survive? I mean, I don't usually browse the archaea aisle at my local supermarket, so where does my B-12 come from? Well, if you eat enough veggies, you can get your B-12 from the soil bacteria clinging to them. Some herbivores can even supply their own B-12 needs, with bacteria living in their stomachs.

But what if you're a carnivore?

Well, do you remember Jurassic Park? How the dinosaurs were engineered with a lysine deficiency. They could live as long as the park staff supplied them with lysine supplements but, if they ever escaped, they were supposed to die. But then in Lost World, they managed to survive without supplements, because the herbivores ate lysine-rich foods and the carnivores ate the now-lysine-rich herbivores.

Only single-celled organisms produce B-12, but they're consumed by herbivores, who are consumed by carnivores.

Did I just blow your fucking mind? I bet I did. Don't even try to deny it. You were all like, "No, it can't be!" And then I was like, "It totally is!" And then you were left standing there with your mind blown.

It was awesome.

If you’re a typical human omnivore, most of your B-12 comes from eating meat, eggs, and/or milk. The animals that produce those products got their B-12 from bacteria, living either on their food or in their digestive tracts.

Humans also possess gut bacteria that produce B-12, but those bacteria live too deep in our digestive system to be very helpful. By the time you get to the colon, where humans keep most of their B-12 producing bacteria, the digestive tract has virtually stopped absorbing B-12. Cows and other ruminants don't have that problem, because they have B-12 producing bacteria right at the front of their digestive tract, in the stomach. And some herbivores, who can't get enough B-12 from soil bacteria and don't have bacteria in their stomachs, have a special trick for accessing the B-12 produced by their colon bacteria: they eat their own poop.

Nature finds a way, but nature never said it wouldn't be disgusting.

Since we're perched at the top of a very precarious pyramid, our bodies have evolved to be very miserly with B-12. We recycle as much as possible, use very little, store enough of it to last for years, and scavenge up every microgram that we get our hands on. Our small intestines are extremely efficient at absorbing B-12, making us one of the animals species that can recover nutritionally significant amounts of B-12 from soil bacteria. That's why vegans don't drop dead from B-12 deficiencies or have to eat their own poop. (Still, B-12 supplements are recommended, especially if you're fastidious about washing your vegetables.)

“But hold on,” I hear you say, “if B-12 is so essential, and we can’t produce it, and it degrades in the stomach, wouldn’t we evolve an effective and elegant mechanism to protect this vital nutrient?”

To which I answer, "What, you think you can write this article better than me? Don't fuck with me. I'll cut you."

As I was saying, we’ve evolved an effective and elegant mechanism to protect this vital nutrient. Haptocorrin is secreted in the saliva and binds to any B-12 it can find, protecting it from acid degradation in the stomach. The stomach, for its part, produces intrinsic factor, which does nothing whatsoever.

At first.

It isn't until all three of them—B-12, haptocorrin, and intrinsic factor—have made it to the safety of the small intestine that intrinsic factor kicks in. Because there, haptocorrin is cleaved by pancreatic enzymes, releasing B-12 from its bonds. The B-12 has a few seconds to enjoy its newfound freedom before intrinsic factor swoops in and binds it up again. With the hand-off complete, intrinsic factor ferries its new prisoner across the walls of the small intestine and into the bloodstream, to be used and abused by all the cells of the body.3

It's the body's clever little way to having its cake and eating it too.

I, for one, am just glad that I don't have to eat my own poop, unlike some people I could mention...